High energy fuel systems



United States Patent 0 3,381,473 HIGH ENERGY FUEL SYSTEMS Donald K. Kuehl, Manchester, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware No Drawing. Filed June 22, 1964, Ser. No. 377,057 28 Claims. (Cl. 60219) This invention relates to metal ignition and more particularly to increasing the rate and decreasing the temperature of ignition of metals on which protective oxide coatings tend to form when the surface thereof is contacted with oxygen and/ or oxygen containing environments.

This invention is particularly useful with fuels or fuel systems comprising or utilizing metals of the type described as a component.

Fuels containing metals treated in accordance with the teachings contained herein are capable of use in a wide variety of transportation and military equipment. They are especially useful as high energy fuels for reaction thrust propellant motors or engines, such as missile, rockets, jet, and ram-jet engines. In motors or engines of the type described, a fuel composition is burned in a combustion chamber and the resulting combustion product admixture is allowed to escape through a jet, thereby producing a driving force.

An object of this invention is to improve the rate and lower the temperature of ignition of metals on which protective oxide coatings form when the surface thereof is contacted with oxygen and/ or oxygen containing environments, e.g., oxygen containing gases, liquids or solids, and includes improving the rate and temperature of ignition of compositions containing such metals.

Another object of this invention is to improve the efficiency of combustion of metals of the type described, including compositions containing such metals.

It is another object of the present invention to provide improved metal additives for solid, liquid, and/or hybrid rocket propellants.

Another object of the present invention is to increase the delivered performance of metal containing propellants by reducing ignition delay.

Another object of this invention is to provide more complete combustion of metal containing fuels in combustion chambers of thrust propellant engines.

Still another object of the present invention is to provide metal additives for incorporation into solid, liquid, or hybrid high energy propellants to improve the performance characteristics thereof.

A further specific object of this invention is to improve the ignition characteristics of beryllium.

Another specific object of the invention is to improve the ignition characteristics of aluminum.

Others objects of the present invention will in part be obvious and will in part appear hereinafter.

It is known that the presence of a protective oxide coating on certain high energy metals prevents ignition of the metals until a much higher temperature is reached than would be required in the absence of such a protective oxide.

It is known that when a protective oxide coating forms on the surface of such metals being heated to ignition temperature in the presence of oxygen or any oxygen containing environment, a temperature approaching or substantially equal to the melting point of the protective metal oxide coating must be reached before ignition will occur. Since the melting point of the protective oxide is usually substantially higher than the ignition temperature of the metal itself, it will readily be appreciated that the presence of the protective oxide considerably hampers the ignition performance of the base metal.

To illustrate this point, the table below compares the "ice melting points of certain high energy metals with the melting point of the oxides of such metals.

It will readily be seen from the table that the melting points of the oxides are substantially higher than the melting points of the metals themselves.

According to this embodiment, there is superimposed ignition characteristics of metals of the type described, i.e., metals which form oxides having a higher melting point than the melting point of the metal itself, may be improved by producing at the surface of the base metal, e. g., as a coating, an alloy or mixture of two or more elements capable of forming, upon oxidation, at least one eutectic composition of mixed oxides of said elements which has a melting point below the melting point of the oxide of the base metal. Preferably, the mixture of elemeats will be capable of forming, upon oxidation, several such eutectic compositions of mixed oxides of said elements.

In a preferred embodiment, the base metal constitutes at least one of the elements in said mixture of elements, as will be made more clear hereinbelow.

According to this embodiment, there is superimposed on the base metal core, as a coating, a mixture or alloy of the base metal itself with another metal, the mixture being capable of forming, upon oxidation, a eutectic composition of mixed oxides having the characteristics described above.

One of the unique advantages of the metals treated in accordance with the teachings contained herein resides in the fact that the treated metals possess an excellent protective coating at room temperature, and therefore do not require special handling or storage equipment.

Although particularly directed to metal containing fuels or fuel systems, this invention is generally useful whenever it is desired to burn or otherwise react metals to produce power, heat or light.

Thus, metals treated in the manner described herein could be used to a good advantage in camera flash bulbs and flares, as well as in other pyrotechnic display devices. The metals or metal particles could also be used in high temperature cutting or welding torches, e.g., oxyacetylene torches. In such torches, the metal particles could be injected into the oxygen stream.

As a further and important application, metals treated in accordance with the teachings contained herein may be used as components in the so-called metallized explosives, i.e., explosives in the nature of dynamite comprising conventional oxidant, e.g., ammonium nitrate, and high energy metal particles.

In all of these applications, "it is the rapid, low temperature ignition properties of the metals of this invention which are exploited to enhance the operational performance and the efiiciency of the devices or compositions containing the metals as components.

Although the invention will 'be particularly described in connection with fuels for thrust propellant application, it should be understood that the basic concepts are equally applicable to other, more general applications of the type described.

When metals which form protective oxide are heated in oxygen containing environment, a protective oxide coating forms and builds up as the temperature is raised to ignition. The protective metal oxide coating adversely affects the performance of the metal, since it results in a longer ignition time, and higher ignition temperature than would be required were the coating not present. In thrust propellant motors utilizing metal containing fuels, the fuel has a relatively short, limited residence time in the combustion chamber of the engine. Protective oxide coatings which form on the metal components of such propellants cause delay in ignition time and high ignition temperatures, thereby leading to incomplete combustion and to reduction in the delivered performance of the fuel.

The use of the metals of this invention as fuel additives in solid, liquid, or hybrid thrust propellant systems will increase the delivered performance through reduction in ignition delay, which will lead in turn to more complete combustion of the metal in the combustion chamber.

Although generally applicable to improving the ignition performance of all metals, which form an oxide having a melting point above the melting point of the metal itself, this invention is particularly directed to enhancing the ignition characteristics of beryllium, aluminum, zirconium, magnesium, silicon, titanium, chromium, including alloys or mixtures of the foregoing, and alloys or mixtures containing at leat one of the foregoing. Particularly advantageous results are achieved when the invention is practiced with: aluminum; beryllium, alloys or mixtures of aluminum and beryllium; alloys or mixtures of any of the listed metals with aluminum; alloys or mixtures of any of the listed metals with beryllium; and alloys of mixtures of any of the listed metals with lithium.

Aluminum and beryllium, including alloys or mixtures comprising beryllium and aluminum, treated as described herein, constitute preferred species of the invention. In this category may also be mentioned alloys of mixtures of aluminum, beryllium, or mixtures of aluminum and beryllium, with lithium.

It will be appreciated that the teachings contained herein are applicable regardless of the form, shape, or size of the metal or alloy being treated.

Typically, however, the metal or alloy will be in a form which will provide a relatively high surface to volume ratio, such as a sphere, cylinder, toroid, and the like.

Ordinarily the metal or alloy will be in the form of spheres, irregularly shaped pieces, wires, strips, and the like, which have, as an inherent characteristic, a minimum size in one direction, e.g., a diameter. As used herein, the term particle or particles, is intended to encompass irregularly shaped pieces, spheres, wires, strips, and the like.

The burning efficiency of metals is a function of many variables. For any given metal, however, the burning efficiency can be generally said to vary inversely with the size of the particles sought to be burnedthe larger the particlesthe lower the burning efficiency.

For rapid burning rate and high burning efficiency, the metal particles should possess, as an inherent characteristic, a small minimum dimension in one direction, e.g., a small diameter, so as to provide a large surface to volume ratio.

One of the advantages of the present invention resides in the fact that it permits utilization in thrust propellant fuel systems of metal particles having a minimum dimension, e.g., diameter, which is larger than has heretofore been possible.

Heretofore in thrust propellant technology, metal particles with a minimum dimension of 0.010 to 0.025 were about the largest that could be tolerated, even with the biggest motors available, because of ignition dimculties. By applying the teachings of this invention, use of metal particles having a minimum dimension of up to 1" or even larger are entirely possible, although particles having a minimum dimension of up to /2 are preferred Metal particles of the type described herein having a minimum dimension ranging from about 0.0001 micron to about A inch constitute preferred embodiments of the invention.

The maximum dimension of the metal particles is not significant. Thus, wires or strips having lengths of up to several miles or even longer are entirely within the purview of the invention and entirely feasible for use with the larger rocket engines.

Metallic particles surface treated in accordance with this invention will ignite more readily in the combustion chamber of a rocket motor and therefore will achieve more complete combustion within the chamber, thereby leading to greater delivered performance. Since the amount of extraneous metals or elements required to produce the surface coating is small, the presence of the coating will not reduce the potential impulse of the base metal appreciably.

In one preferred embodiment of this invention, beryllium is provided with a surface alloy of aluminum-beryllium, to thereby lower the ignition temperature of the beryllium.

The ignition of beryllium at conveniently low temperatures is prevented by the presence of a protective oxide of BeO which melts at a temperature higher than 2500 C. See table, supra. It has been discovered that mixtures of aluminum oxide and beryllium oxide containing between about 35 mole percent and mole percent A1 0 melt at considerably lower temperatures, i.e., below 2000 C. In application requiring the combustion of beryllium, where ignition at as low a temperature as possible is desired, the presence of a mixed oxide coating melting much lower than BeO will therefore facilitate ignition. An alloy of beryllium-aluminum in the proper proportions to form the desired mixed oxide on complete reaction should contain 75 to 99% aluminum by weight. This, however, is more aluminum than desired to give optimum heat release on reaction. Therefore, according to the preferred embodiment of this invention, only the outer layer of beryllium is alloyed to the required aluminum concentration.

To surface alloy the beryllium with the required aluminum concentration, a thin layer of aluminum is deposited on the beryllium base metal, as by vapor deposition or similar metal deposition techniques. The amount of aluminum deposited will ordinarily amount to less than 20 percent of the weight of beryllium.

After deposition, the entire combination of aluminum layer on beryllium substrate or core is heated to between 700 C. and 1100 C. in an inert atmosphere to fuse the aluminum and form an alloy of the desired composition on the surface of the beryllium core.

For example, assuming the beryllium to be spherical in shape, aluminum could be deposited to a depth equal to about of 1 percent of the diameter of the sphere, or about 1 percent by weight aluminum. On heating to 900 C., sufficient beryllium would dissolve in the aluminum to form a 5 percent by weight beryllium alloy. On oxidation, this alloy layer would oxidize to a mixed oxide melting at around 1900 C., exposing the base beryllium for ignition. If the original treated particle were heated first to 1040 C., enough beryllium would dissolve in the aluminum to form a 10 percent by weight beryllium alloy. The oxidation products of this alloy would melt at about 1850 C. Initial heating to about 1090 C. will form an alloy coating whose oxidation products melt at about 1835" C., or about 700 C. lower than BeO.

The aluminum-alloy coated beryllium product produced in the manner described above ignites at a lower temperature than pure beryllium, but still has an excellent protective oxide coating at room temperature which facilitates handling and storage. Because of the small amount of aluminum required, there will be very small theoretical performance losses compared with pure beryllium, when the aluminum-alloy coated beryllium particles are utilized as a fuel in a thrust propellant.

Note that in the fusion step, the temperature to which the aluminum clad beryllium core is heated in an inert environment determines the amount of beryllium which is dissolved in the aluminum cladding, and hence the composition of the alloy which constitutes coating on the beryllium core. This is so because at any given temperature, enough beryllium will dissolve in the aluminum to form a saturated solution of beryllium in aluminum at the given temperature. This alloy coating, as indicated above, should contain between 75 and 99 percent aluminum by weight.

Using the same procedure described above, a new and useful product comprising aluminum as a core and a mixture of aluminum and beryllium as a coating may be made. Such a product constitutes a second preferred species of the product of this invention. In making such a product, an aluminum core is clad with a thin layer of beryllium, as by vapor deposition or other suitable coating techniques. The resulting beryllium clad aluminum core is then heated in an inert environment to form an alloy of the desired composition on the aluminum core. Here again, the weight of the beryllium cladding will be less than 50 percent and usually less than 20 percent of the weight of the aluminum core. Preferably, the beryllium cladding will amount to about 1 percent or less of the weight of the aluminum.

As will be clear from the foregoing, a particularly suitable technique for making the metal particles of this invention is to coat the base metal with a thin coating of the alloying element. The alloying element clad base metal core is then heated in an inert atmosphere to a temperature which will fuse the cladding element to form an alloy with the base metal which has the desired composition, i.e., that composition which will, upon oxidation, form a eutectic composition of mixed oxides which melts below the melting point of the pure oxides. By an inert atmosphere is meant 'one that is substantially free of oxide, and includes vacuum, and inert gases, such as nitrogen, neon, helium, krypton, and the like.

Metal treated in accordance with this invention can be used in the solid grain of a hybrid or tripropellant rocket to provide improved performance. It may also be used in a slurry or gel of either fluid in a bipropellant liquid rocket.

Use of the surface treated metals of this invention in solid rockets will be advantageous, since no special precautions need be taken to prevent reaction of the metal with the solid state oxidizer during mixing, curing or storage.

Besides the metal, rocket propellants will comprise an oxidizer or utilize an oxidizer as a component. By oxidizer is means oxygen or a chemical capable of being decomposed to yield oxygen. When solid, the fuel will comprise a binder. Other constituents of rocket propellants are well known in this art. Fuels, i.e., chemical substances capable of entering into chemical reactions to release heat and/ or gases, other than the metal component may also be used. It should be understood that the teachings contained herein are equally applicable to any propellant system which utilizes metal as a high energy additive.

Typical rocket propellant systems in which metals treated as described herein can be used to good advantage are described in United States Patent Nos. 3,133,842 and 3,112,609. Berryllium which has been surface alloyed with aluminum finds special application in the tripropellant H 0 Be system described, for example, in United States Patent No. 3,112,609.

The invention in its broader aspects is not limited to the specific details shown and described but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. In a high energy fuel system, the improvement which includes as a component, a high energy additive which comprises a core metal selected from the group consisting of beryllium, aluminum, zirconium, magnesium, silicon,

titanium, chromium, including mixtures and alloys of the foregoing, said core metal being characterized by the ability to form an oxide having a melting point above the ignition temperature of the metal itself under ordinary conditions of storage, a coating on said core metal which is a eutectic composition of mixed oxides having a melting point below the melting point of the oxide of said core metal, said coating being characterized by the ability to protect said core metal at room temperature under ordinary conditions of storage, and being itself stable under the aforesaid conditions.

2. The improvement of claim 1 wherein one of said oxides making up the eutectic composition is an oxide of said core metal.

3. The high energy vfuel system of claim 1 wherein said core metal comprises lithium.

4. In a high energy fuel system, the improvement which includes, as a component, a metal additive which comprises a core metal selected from the, group consisting of beryllium, aluminum, mixtures and alloys of the foregoing and mixtures and alloys comprising one of the foregoing, said core metal having a coating which is a eutectic com-position comprising a mixture of oxides having a melting point below the melting point of the oxide of core metal, said coating being characterized by an ability to protect the core metal at room temperature under ordinary conditions of storage, and being itself, stable under the aforesaid conditions.

5. The improvement of claim 4 wherein said coating comprises a eutectic composition comprising a mixture of beryllium oxide and aluminum oxide.

6. The improvement of claim 4 wherein said core metal is aluminum, and wherein said coating is a eutectic com position comprising a mixture of beryllium oxide and aluminum oxide.

7. The improvement of claim 4 wherein said core metal is beryllium, and the coating is a eutectic composition comprising a mixture of aluminum oxide and beryllium oxide.

8. The improvement of claim 5 wherein the mixture of aluminum and beryllium at the surface of said member contains about to 99 percent aluminum by weight based upon the metal content of said coating. 9. A method for reducing the ignition time and lowering the ignition temperature of a high energy metal Which forms an oxide having an ignition temperature above the melting point of the metal, which comprises the steps of establishing a core of said metal, superimposing on said core at least one element capable of alloying with said metal, heating the resulting structure in an inert atmosphere to a temperature high enough to form an alloy of said metal and said element at the surface of said core, said alloy being capable of forming, upon oxidation, a eutectic composition of mixed oxides of said metal and said element which has a melting point below the melting point of the oxide of said metal and exposing the resulting metal to an oxygen containing environment to thereby form on the metal a coating which is a eutectic composition comprising a mixture of the oxides of said high energy metal and said element, which has a melting point below the melting point of the high energy metal, said coating characterized by the ability to protect the high energy metal at ordinary temperature and under ordinary storage conditions, and being itself stable under the aforesaid conditions.

10. The method of claim 9 wherein said metal is a member selected from the group consisting of beryllium, aluminum, zirconium, magnesium, silicon, titanium, chromium, mixtures of the foregoing, and mixtures comprising any of the foregoing.

11. The method of claim 9 wherein said metal is beryllium and said element is aluminum.

12. The method of claim 9 wherein said metal is aluminum and said element is beryllium.

13. A method for reducing the ignition time and lowering the ignition temperature of beryllium which comprises forming on the surface of beryllium a deposit of aluminum, heating the aluminum coated beryllium in an inert environment to a temperature high enough to fuse the beryllium with the aluminum and to form on the beryllium an aluminum-beryllium alloy containing 75 to 99 percent aluminum by weight based upon the metal content of said coating, said alloy being capable upon oxidation of forming a eutectic composition of mixed oxides of aluminum and beryllium which has a melting point below the melting point of beryllium oxide.

14. A method for reducing the ignition time and lowering the ignition temperature of aluminum which comprises forming on the surface of aluminum a deposit of beryllium, heating the beryllium coated aluminum in an inert environment to a temperature high enough to fuse the beryllium with the aluminum to form on the aluminum an aluminum-beryllium alloy containing 75 to 99 percent aluminum by weight, said alloy being capable upon oxidation of forming a eutectic composition of mixed oxides of aluminum and beryllium which has a melting point below the melting point of aluminum oxide.

15. In a method of operating a jet combustion engine, wherein fuel comprising metal as a component is burned in a combustion chamber and the combustion product admixture is allowed to escape through a jet, thereby producing a driving force, the improvement which comprises utilizing, as the metal component, a metal which forms an oxide having a melting point above the melting point of the metal, the surface of said metal comprising a mixture of elements, said mixture of elements being capable of forming, upon oxidation, a eutectic composition of mixed oxides of said elements which has a melting point below the melting point of the oxide of said metal said eutectic composition being characterized by the ability to protect said core metal at room temperature under ordinary conditions of storage, and being itself stable under the afore said conditions.

16. The improvement of claim 15 wherein one of said mixture of elements is said metal.

17. The improvement of claim 15 wherein said metal is a member selected from the group consisting of beryllium, aluminum, zirconium, magnesium, silicon, titanium, chromium, mixtures of the foregoing, and mixtures comprising any of the foregoing.

18. The improvement of claim 15 wherein the base metal is beryllium and the mixture of elements is a mixture of beryllium and aluminum containing 75 to 99 percent aluminum by weight based upon the metal content of said coating.

19. The improvement of claim 15 wherein the base metal is aluminum and the mixture of elements is a mixture of beryllium and aluminum containing 75 to 99 percent aluminum by weight.

20. As a new composition of matter, a high energy particle having a core which is a member selected from the group consisting of beryllium, aluminum, zirconium, magnesium, silicon, titanium, chromium, mixtures of the foregoing, and mixtures comprising any of the foregong, the surface of said core comprising a mixture of elements capable of forming, upon oxidation, 2. eutectic composition of mixed oxides of said elements which has a melting point below the melting point of the oxide of said member said eutectic composition being characterized by the ability to protect said core metal at room temperature under ordinary conditions of storage, and being itself stable under the aforesaid conditions.

21. As a new composition of matter, a high energy particle comprising a core which is a member selected from the group consisting of beryllium and aluminum, the surface of said core comprising a mixture of elements, capable of forming, upon oxidation, a eutectic composition of mixed oxides of said elements which has a melting point below the melting point of the oxide of said member said eutectic composition being characterized by the ability to protect said core metal at room temperature under ordinary conditions of storage, and being itself stable under the aforesaid conditions.

22. As a new composition of matter, a high energy particle comprising a core which is a member selected from the group consisting of aluminum and beryllium, the surface of said core being coated with an alloy of beryllium and aluminum which contains 75 to 99 percent aluminum based upon the metal content of said coating.

23. The metal of claim 20 in the form of a particle having a minimum dimension less than 1 inch.

24. The metal of claim 20 in the form of a particle having a minimum dimension between about 0.001 micron and A inch.

25. The method of claim It) wherein said core metal comprises lithium.

26. The improvement of claim 17 wherein said core metal comprises lithium.

27. The improvement of claim 20 wherein said core metal comprises lithium.

28. The improvement of claim 22 wherein said core metal comprises lithium.

References Cited UNITED STATES PATENTS 2,968,917 l/l961 Whaley 6035.4 3,070,469 12/1962 Jenkins 1495 3,071,493 l/l963 Whaley et al 117-131 X 3,133,841 5/1964 Kuehl 149-44 X CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

S. I. LECHERT, .lR., Assistant Examiner. 

15. IN A METHOD OF OPERATING A JET COMBUSTION ENGINE, WHEREIN FUEL COMPRISING METAL AS A COMPONENT IS BURNED IN A COMBUSTION CHAMBER AND THE COMBUSTION PRODUCT ADMIXTURE IS ALLOWED TO ESCAPE THROUGH A JET, THEREBY PRODUCING A DRIVING FORCE, THE IMPROVEMENT WHICH COMPRISES UTILIZING, AS THE METAL COMPONENT, A METAL WHICH FORMS AN OXIDE HAVING A MELTING POINT ABOVE THE MELTING POINT OF THE METAL, THE SURFACE OF SAID METAL COMPRISING A MIXTURE OF ELEMENTS, SAID MIXTURE OF ELEMENTS BEING CAPABLE OF FORMING, UPON OXIDATION, A EUTECTIC COMPOSITION OF MIXED OXIDES OF SAID ELEMENTS WHICH HAS A MELTING POINT BELOW THE MELTING POINT OF THE OXIDE OF SAID METAL SAID EUTECTIC COMPOSITION BEING CHARACTERIZED BY THE ABILITY TO PROTECT SAID CORE METAL AT ROOM TEMPERATURE UNDER ORDINARY CONDITIONS OF STORAGE, AND BEING ITSELF STABLE UNDER THE AFORESAID CONDITIONS. 